Class I YSOs possess a very strong IR excess (flat or positive spectral indices), are embedded, and are brighter in the near- and mid-IR bands. There is some statistical evidence that Class I YSOs - protostars - are the progenitors of TTSs; they are typically less abundant than Class II YSOs, are more likely to coincide with dark clouds and molecular cores, and may be much younger (< 100,000 yr).

T Tauri stars are known to vary on periods of days, weeks, and months. Causes include episodic accretion events, warped disk occultations, the transit of dark spots in the photosphere.

Relevant characteristics of TTSs

irregular and quasi-periodic variations

high amplitude (up to 5 mag)

may have a phase of quiescence (see figure)

absolute magnitude range: M_V=? to ? mag

typically do not require a trigger, but watch list of T Tauris would be useful

Spectral features

FU-Ori, EX-Ori and other outbursts

Some young pre-main-sequence stars have exhibited quite drastic episodic variations.
1-3 mag eruptions have been observed in several objects (e.g. EX Lup, DR Tau), while
more spectacular eruptions corresponding to a brightening of 3-6 mag have been measured
in FU Orionis type variables (FU-ors). A dozen young stars are now characterised as FUors. Most have
been observed to rise 3-5 mag in brightness in less than one
year; V1515 Cyg is the only known example to require a decade to rise
to visual maximum. Note, however, that the historical light curves for many systems
are poorly documented.